The present invention relates generally to the electrochemical detection of substances in solution and more particularly to an improved method and means for calibrating sensing electrodes.
The measurement of substances in solution has long been determined by known techniques involving voltametric and amperemetric cells in contact with the solution to be tested. These cells act as sensing electrodes which produce voltages and currents respectively, which are proportional to the level of substance being detected. A suitable voltameter or ammeter is employed to indicate the level of substance measured.
A number of sensing electrodes are employed by those skilled in the art to measure a variety of cations, anions, dissolved gases, oxidizing and reducing substances. The well known glass pH half-cell used in conjunction with a reference half-cell, referred to herein as a combination pH electrode, which responds reversibly to the pH of a solution according to the Nernst equation, is an example. Another electrode of the general type with which this invention is concerned is the inert metal half-cell/reference half-cell, herein referred to as a combination redox electrode, which measures the oxidizing or reducing power of a solution. Yet another electrode of the general type relevant to this invention is the dissolved oxygen electrode, which produces a current in proportion to the level of dissolved oxygen in solution. Indeed, it will become apparent that the invention is not restricted to use with any particular electrode, and may be used with all types.
All of the above mentioned devices require periodic calibration to correct changes in performance or to ascertain electrode malfunction. Generally one or two solutions of known composition with respect to a given substance are employed to establish substantial compliance with the ideal cell characteristics. Calibration controls on the indicating meter are used to correct deviations.
Those acquainted with the art know that the preferred calibration solutions are those which do not change value appreciably on dilution or contamination. Such solutions are known as buffers, and the capacity of such a solution to resist effects which cause changes in value is termed buffer capacity.
It is also known in the prior art that calibration may be manual or automatic. Manual calibration necessitates the removal of the sensing electrode from the test environment and placement of the sensing electrode in the calibration solution. Automatic calibration has been accomplished by the use of a flow through cell, reservoir of calibration solution, timer and externally operated valve or valves to introduce the calibration solution.
In applications where the sensing electrode monitors a test solution flowing past the electode, certain difficulties arise with the calibration methods of the prior art. The applications of flow referred to herein and generally concerned with this invention are flows of test solution through hollow structures, generally referred to as pipes or tubes, as used in the transference of a body of solution from one space to another, or in the circulation of a fixed body of test solution. The latter application includes such as swimming pools, which employ a pump and filter to circulate the water contained in the swimming pool.
It is found that manual calibration has the disadvantage of requiring the presence of an experienced operator. In addition, removal of the sensing electrode for calibration can be hazardous if the test solution is under pressure or of a corrosive nature. It is also found that mishandling the sensing electrode results in loss of sensing ability.
A fuller understanding of the limitations of the automatic calibration method of the prior art can be gained by consideration of the following description of operation.
When the flow of test solution is stopped to allow calibration, the space surrounding the sensing tip of the electrode must be isolated from the test solution body to allow voiding and introduction of the calibration solution. This normally requires one or two externally operated valves which are capable of resisting the test solution pressure. These valves operate in conjunction with a timing device to perform calibration at prescribed intervals.
It is apparent that such devices overcome the difficulties associated with manual calibration, but are cumbersome and complicated, and are seldom used in practice.